Because biomass fuel contains a low N content and a high volatile matter content, combustion with low NOx and low unburned combustibles emission can be achieved by mixed combustion or combined combustion of biomass and fossil fuel such as coal. A combustion technique using woody biomass as secondary fuel has recently attracted attention as one of measures to reduce CO2 emissions in a fossil fuel combustion boiler.
There are a lot of conventional instances of the woody biomass mixed combustion technique particularly in Europe and North America. There is a method in which woody biomass is mixedly put into an existing coal pulverizer and pulverized and then put together with powdered coal into a boiler furnace from a burner. A method of feeding woody biomass onto a coal-carrying conveyor and mixing and pulverizing the woody biomass together with coal while using a pulverization combustion system in common with coal is generally used domestically in Japan because the cost thereof is lowest.
Pulverized and pelletized woody biomass or under-50 mm pulverized and chipped woody biomass is used as woody biomass on this occasion. As another example of mixed combustion, there is a technique of pulverizing woody biomass independently, feeding the woody biomass to a powdered coal carrying line, mixing the woody biomass with powdered coal and burning the mixture of the woody biomass and the powdered coal in a furnace.
Applicability of low water content and high energy density pellets or briquettes in place of woody chips as fuel for power generation has been discussed recently. This is because pellets or briquettes are not only low in transportation fee but also excellent in storability although the production cost of the pellets or briquettes as fuel is higher than that of pulverized green wood in terms of the cost of raw material production.
FIG. 22 is a schematic configuration view of a conventional roller type vertical pulverization device. The roller type vertical pulverization device is mainly constituted by a drive portion, a pressurization portion, a pulverization portion, and a classification portion.
The drive portion has a mechanism to transmit rotational force from a pulverization portion drive motor 1 placed outside the roller type pulverization device to a speed reducer 2 and transmitting the rotational force of the speed reducer 2 to a rotary table 3 placed on top of the speed reducer 2.
A pressurization frame 6 placed inside the roller type pulverization device is pulled down through a rod 5 by a hydraulic cylinder 4 placed outside the roller type pulverization device, so that the pressurization portion can apply a pulverization load to a bracket 7 paced at the bottom of the pressurization frame 6.
In the pulverization portion, pulverization rollers 8 disposed at circumferentially regular intervals on the rotary table 3 are supported by the pressurization frame 6 and the bracket 7. The pulverization rollers 8 rotate according to rotation of the rotary table 3, so that a pulverization target 10 put through a raw material feed pipe 9 is pulverized by nip portions between the rotary table 3 and the pulverization rollers 8.
The classification portion has a cyclone type fixed classifier 12 provided with fixed classification fins 11, and a rotating classifier 14 provided with rotary classification fins 13. A recovery cone 15 is attached to lower end portions of the fixed classification fins 11. As shown in the drawing, the rotating classifier 14 is disposed inside the fixed classifier 12 to thereby provide a double classification mechanism. The rotary classification fins 13 are driven to rotate by a classification motor 24 through a hollow rotary shaft 23 disposed on an outer circumference of the raw material feed pipe 9.
The pulverization target 10 such as coal put through the raw material feed pipe 9 falls down to a central portion of the rotating rotary table 3 and moves to the outer circumferential side of the rotary table 3 with a spiral locus drawn on the rotary table 3 by centrifugal force generated in accordance with the rotation of the rotary table 3, so that the pulverization target 10 is nipped and pulverized between the rotary table 3 and the pulverization rollers 8 rolling thereon.
The pulverized pulverization target 10 further moves to the outer circumference and meets with a carrying gas 18 such as high-temperature primary air introduced into a mill casing 17 from a throat 16 provided on the outer circumference of the rotary table 3, so that the pulverized matter is blown up while dried.
A section from the throat 16 to the lower end of the fixed classifier 12 is called primary classification portion. The blown-up pulverized matter 19 is classified by gravitation so that coarse particles fall down and are returned to the pulverization portion.
The fine pulverized matter 19 which has reached the classification portion is classified into fine particles 20 not larger than a predetermined particle size and coarse particles 21 larger than the predetermined particle size by the fixed classifier 12 and the rotating classifier 14 (secondary classification). The coarse particles 21 fall down along the inner surface of the recovery cone 15 and re-pulverized. On the other hand, the fine particles 20 are carried by an air flow to a destination such as a coal-fired boiler (not shown) via a feed pipe 22.
FIG. 23 is a partly enlarged schematic configuration view of the classification device provided in the conventional roller type pulverization device.
As shown in the drawing, the rotary classification fins 13 are disposed inside the fixed classification fins 11 and fixed and supported to a lower ring support 25 and an upper ring support 26 so that the rotary classification fins 13 are put between the two ring supports 25 and 26. The lower ring support 25 and the upper ring support 26 are connected to each other with a distance on the outer circumferential side of the rotary shaft 23 (see FIG. 22), so that the rotary classification fins 13, the lower ring support 25 and the upper ring support 26 rotate integrally with the rotary shaft 23.
The planar shape of each rotary classification fin 13 is rectangular. A large number of rotary classification fins 13 are set at regular intervals along the circumferential direction of the ring supports 25 and 26 so that the width direction of each rotary classification fin 13 faces the rotation center of the rotating classifier 14 (see FIG. 22).
A narrow gap (narrow portion 28) is formed between the upper ring support 26 and a top plate 27 provided above the upper ring support 26. The narrow portion 28 is a gap which is provided so that the upper ring support 26 is prevented from coming into contact with the top plate 27 even if the rotating classifier 14 rotates. If the narrow portion 28 is tall, that is, if the gap between the upper ring support 26 and the top plate 27 is large, there is a possibility that the coarse particles 21 may pass through the gap so as to be mixed with the classified fine particles 20. For this reason, it is impossible to make the narrow portion 28 too tall, so that the size of the gap (narrow portion 28) between the upper ring support 26 and the top plate 27 has to be set strictly to be several millimeters, compared with the upper ring support 26 (rotary classification fins 13) having a huge outer diameter.